Alkali metal catalysts for product gas tar reduction have also been studied. However, most of the work carried out in this field focuses on the effect of the catalysts on the gasification and pyrolysis reactions. The alkali metal catalysts are often used as primary catalysts, added directly to the biomass by dry mixing or wet impregnation. When directly added to the gasifier bed, these catalysts tend to increase the rate of gasification, reduce the tar content (sometimes significantly) and reduce the methane content of the product gas (Sutton, 2001). However, they tend also to be rapidly deactivated and the recovery of the catalysts is difficult and costly.

Inherent in the ash of several biomass types are the high concentrations of alkali and these ashes are effective catalysts for tar decomposition (Hauserman, 1994).

Alkali metal catalysts are also active as secondary catalysts. For example, potassium carbonate supported on alumina is more resistant to carbon deposition than nickel, although it is not as active.